{"id":93322,"date":"2026-01-10T13:54:57","date_gmt":"2026-01-10T12:54:57","guid":{"rendered":"https:\/\/www.telescoop.nl\/?post_type=product&p=93322"},"modified":"2026-09-04T14:05:00","modified_gmt":"2026-09-04T12:05:00","slug":"omegon-main-mirror-304mm-f5-mirror-set","status":"publish","type":"product","link":"https:\/\/www.telescoop.nl\/en\/omegon-hoofdspiegel-304mm-f5-spiegelset\/","title":{"rendered":"Omegon Main Mirror 304 mm f\/5 Mirror Set"},"content":{"rendered":"

Omegon Primary Mirror 304 mm f\/5 Mirror Set: optics for a self-built 12-inch Newtonian<\/strong><\/h2>\n\n\n\n

Anyone who builds a large Dobson or Newtonian telescope themselves starts with the mirrors. The rest of the telescope, the tube or truss, the rocker box, and the focuser, is woodwork and mechanics that you can make yourself. The optics are not: they determine what the instrument will ultimately show.<\/p>\n\n\n\n

This set supplies both mirrors you need. The 304 mm parabolic primary mirror and the elliptical secondary mirror are matched to each other, so you don't have to calculate the size of the secondary mirror yourself.<\/p>\n\n\n\n

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Parabolic, not spherical<\/h4>\n\n\n\n

At f\/5 a parabolic mirror is necessary. A spherical mirror at this ratio would produce visible spherical aberration, preventing stars from ever becoming truly sharp.<\/p>\n\n\n\n

The primary mirror is made of borosilicate glass. It expands less than regular window glass, so the mirror retains its shape after cooling.<\/p>\n<\/div>\n<\/div>\n\n\n\n

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Secondary mirror with small 70 mm minor axis<\/h4>\n\n\n\n

The elliptical secondary mirror has a minor axis of 70 mm. That size was chosen as a balance between two things: large enough to fully illuminate the field of view, small enough to limit the central obstruction.<\/p>\n\n\n\n

A smaller secondary mirror yields higher contrast on planets, while a larger one illuminates a larger photographic field. This size suits the versatile use for which an f\/5 Newtonian is intended.<\/p>\n<\/div>\n<\/div>\n\n\n\n

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What f\/5 means in practice<\/h4>\n\n\n\n

With a 304 mm aperture at f\/5, you get a focal length of 1500 mm. That is short for this aperture, which keeps the tube manageable and yields a wide field of view.<\/p>\n\n\n\n

The flip side is that a short f-ratio shows coma at the edge of the image. For visual work that is not too bad, for photography a coma corrector is recommended.<\/p>\n<\/div>\n<\/div>\n\n\n\n

Included<\/h3>\n\n\n\n